Paracyclophanes and in particular [2.2]-paracyclophane derivatives are established ligands for transition metal-catalysed asymmetric reactions (see for example, S. E. Gibson and J. D. Knight, Org. Biomol. Chem., 2003, 1, 1256-1269). Of these, paracyclophane bis(phosphines) have attracted considerable attention because catalysts derived from them show high levels of activity and selectivity in a number of useful asymmetric transformations.
For example, WO 97/47632 describes paracyclophane bis(phosphine) ligands and rhodium (Rh), ruthenium (Ru), iridium (Ir) or palladium (Pd) catalysts derived therefrom for asymmetric hydrogenation, isomerization, hydroboration, cyclization, arylation, alkylation and amination reactions. The ligands described have the formula depicted below;

Where both X groups are the identical, these ligands posses C2 symmetry, that is they are chiral and have a C2 axis of symmetry. For example, the C2-symmetric [2.2]ligand where X=—(CH2CH2)—, known as PHANEPHOS, may be used in the asymmetric hydrogenation of ketones when comprising part of a Ru-diamine complex (see WO 01/74829).
WO 02/057278 describes paracyclophane ligands structurally related to the paracyclophane bis(phosphines) where the phenyl groups bound to the phosphorus in the [2.2]paracyclophane structure are replaced by oxygen, nitrogen, chloride or hydrogen atoms. These ligands are depicted below;

Rh, Ir and Ru catalysts derived therefrom were used in asymmetric hydrogenation reactions.
WO 2004/111065 describes substituted paracyclophanes of formula (I)
wherein X1 and X2 are linking groups comprising between 2 to 4 carbon atoms, Y1 and Y2 are selected from the group consisting of hydrogen, halide, oxygen, nitrogen, alkyl, cycloalkyl, aryl or heteroaryl, Z1, Z2 and Z3 are substituting groups that optionally contain functional groups, a, b, c, d, e and f are 0 or 1 and a+b+c+d+e+f=1 to 6. Preferably X1 and X2 are —(C2H4)—and a+b+c+d+e+f=1 or 2.
Whereas the paracyclophane ligands described are effective for many asymmetric transformations there is still a need to improve the activity and selectivity of catalysts derived from them over a broader range of reactions and substrates.
The above ligands rely on the dual functionality provided by the two phosphorus atoms to provide the desired selectivity. We have found surprisingly that certain paracyclophanes comprising only one phosphorus moiety are useful ligands.